The present invention relates generally to heat pumps and air-conditioning equipment. More particularly, the invention relates to a system for diagnosing a blocked fan condition in the outdoor heat exchanger.
Conventional air-conditioning and heat pump systems transfer energy from a low temperature air source to a high temperature air sink by pumping refrigerant through a closed circuit which includes an indoor heat exchanger or coil and an outdoor heat exchanger or coil. In the air-conditioning or cooling mode the heat pump extracts heat from the inside air and releases it into the outside air. In the heating mode the process is reversed. The heat pump extracts heat from the outside air and releases it into the living space. Thus in the cooling mode the indoor heat exchanger communicates with the low temperature air source and the outdoor heat exchanger with the high temperature sink. Conversely, in the heating mode the outdoor heat exchanger communicates with the low temperature air source and the indoor heat exchanger with the high temperature sink. In both cases, heat is transferred or pumped by vaporizing the liquid refrigerant using heat transferred from the air flowing through the heat exchanger at the low temperature source.
Efficient operation of the refrigeration cycle requires that sufficient heat be transferred from the low temperature air source through the associated heat exchanger in order to maintain adequate capacity for meeting the heat transfer demand. Most systems use fans or blowers to force air through the heat exchangers to transfer heat to the various rooms of the building via the circulating air.
Ideally, to achieve optimal system performance, both heat exchangers should be kept free of frost and other foreign matter and the associated fans or blowers and associated air ducts should be kept free of blockage as well. When the outdoor heat exchanger coil takes on an accumulation of frost, or when the airflow passageways or fan become blocked with foreign matter, system performance deteriorates.
Most conventional heat pumps have some mechanism for addressing the frost accumulation problem of the outdoor coil. Typically this is done by reversing the normal cycle, switching from heating mode to cooling mode, for a sufficient time to melt any accumulated frost. In this reverse cycle defrost process, high temperature refrigerant from the discharge of the compressor is redirected to the frosted coil to thereby raise the coil temperature and melt the frost. Unfortunately, such reverse cycle processing decreases system efficiency, since the reverse cycle process causes the system to temporarily act as an air conditioner during the winter and as a heater during the summer. This loss of efficiency is further exacerbated in the case of the heat pump in heating mode. Many heat pump systems rely on auxiliary electric resistance heaters to supplement the heat provided by the heat pump. During the defrost cycle, when the heat pump is acting as an air conditioner, it is often necessary to use the auxiliary electric resistance heaters to offset the cooling caused by the defrost cycle. This further reduces system efficiency.
During the heating operation of the heat pump frost accumulation on the evaporator coil is a common occurrence and the efficiency-degrading defrost cycle is a necessary evil. However, frost accumulation is only one cause of performance degradation. Obstructions in the airflow passages and blockage or partial blockage of the evaporator (outdoor) fan will also degrade system performance. Unfortunately, blockage by foreign matter, such as dust and dirt, leaves, paper and the like cannot be cleared by melting in a defrost cycle. To make matters worse, obstruction of airflow passages or blockage of the outdoor fan will often simulate frost accumulation conditions, invoking unnecessary (and ineffective) defrost cycles. For example, a demand defrost system which relies on sensing air pressure drop across the heat exchanger coil in order to sense the presence of frost accumulation, would be unable to distinguish frost accumulation from foreign matter debris accumulation. Such a system would respond by repeatedly invoking efficiency degrading defrost cycles to no avail.
The problem of airflow passageway obstruction and fan blockage has not heretofore been adequately dealt with. Because outdoor heat exchangers tend to be hidden among foliage in many residential settings, fan and airway blockage is a common problem which often goes undetected.
The present invention addresses the airflow and fan blockage problem by providing a blockage detection system which may be implemented in conjunction with a frost detection system, without requiring expensive modifications or additional sensors. The system of the invention is capable of detecting a fan blockage condition from frost or airway restriction condition due to foreign matter or debris. Thus the system will not repeatedly invoke unnecessary defrost cycles which are incapable of clearing the obstruction. Furthermore, this diagnostic is expected to save energy.
In accordance with one aspect of the invention a blocked fan detection method is provided for detecting an airflow obstruction condition in a heat exchanger. The method comprises determining a temperature difference parameter indicative of the temperature difference between the heat exchanger and the ambient air surrounding the heat exchanger. A defrost cycle is performed when the temperature difference parameter exceeds a predetermined value. After the defrost cycle has completed the temperature difference parameter is checked during a second cycle to validate the diagnosis. A blocked fan or airflow obstruction condition is declared if the second determined temperature difference parameter exceeds a predetermined value.
The airflow obstruction or blocked fan detection system of the invention may be readily integrated with the existing demand defrost logic. In this way, a blocked fan or obstructed airflow condition can be detected as an adjunct to the normal defrost cycle. This allows the system to override or prevent efficiency-degrading defrost operations when the results of which would be of minimal value due to foreign matter blockage. The detection system of the invention can be readily and economically implemented without the need for special sensors. The system works using simple temperature measurements from sensors which are, in most cases, already provided in the heat pump system or which may be inexpensively added.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.